1. Field of the Invention
The present invention relates to signals, in particular, Light Emitting Diode (LED) Signals. More specifically, the present invention relates to an LED traffic signal that is less susceptible to the xe2x80x9csun phantomxe2x80x9d effect, having an improved viewing aspect and a LED light output degradation sensing circuit and light pipe collector, as well as materials, manufacturing and installation cost advantages.
2. Description of the Related Art
LED traffic signals present numerous advantages over common incandescent lamp traffic signals. Use of LEDs provides a power consumption savings and extremely long life in comparison to common incandescent light sources. The long life span creates improved reliability and sharply lowered maintenance costs.
As an individual LED is not bright enough to equal the light output of an incandescent lamp, multiple LEDs are used. Previously, multiple LEDs created a display aspect with multiple individual points of light readily discernible by the viewer. A non-uniform display aspect is commercially undesirable for traffic signals. One method of preventing discernable individual light points has been to use a full array of LEDs. However this is not commercially competitive as each additional LED is a significant percentage of the signals total cost. Each generation of LEDs is becoming brighter and brighter requiring fewer and fewer LEDs to equal the light output of an incandescent lamp but at the same time increasing the likelihood that the individual point sources and/or shadows between each LED are then detectable by the viewer.
Common LEDs include a semiconductor diode pellet located above a cup shaped reflector incased in a barrel shaped epoxy housing with a lens formed in its top. The LED lens and the reflector cooperate to direct approximately 65% of the light emitted by the diode through the lens shaped top end in a vertically directed wide conical light distribution pattern. The remaining 35% of light emitted by the diode is unusable as it is radiated at angles ranging between just outside of the forward conical light distribution pattern and horizontally through the sides of the LED housing.
Attempts to recover and utilize the horizontally radiated light have included mirrored reflectors and/or mirrored optical receiving bodies with mirror coatings on their outside surfaces, thereby creating a parabolic reflector that captures and redirects the horizontal light. Unfortunately, these solutions add more cost than merely adding additional LEDs to make up for the unutilized light. Solutions directed to modifying each individual LED with its own reflector add an additional manufacturing cost to each and every LED. Further, the past solutions for multiple LED embodiments have been tied to a fixed set of LEDs. As LED efficiency increases, the required number of LEDs for a given light output decreases. With each new generation of higher efficiency LEDs, the past solutions require the redesign and remanufacture of the mirrored reflectors, adding further costs to the final product.
Total internal reflection is a phenomenon where electromagnetic radiation (light) in a given medium (for example acrylic or polycarbonate material) incident on the boundary with a less dense medium (for example air), at an angle equal to or larger than the critical angle, is completely reflected from the boundary. Commonly used in fiber optics technology and binocular prisms, properly designed optical components using total internal reflection do not require expensive mirrored surfaces to redirect light. Total internal reflection is described in detail in xe2x80x9cModern Optical Engineeringxe2x80x9d Library of Congress Catalog Card Number 66-18214, hereby incorporated by reference. Applicant is unaware of previous application of total internal reflection as a means for collecting and redirecting horizontal light xe2x80x9clostxe2x80x9d from a common LED.
Due to the large installed base, worldwide, of incandescent traffic signal systems, most LED traffic signals are designed to be retrofitted into existing traffic signal systems originally designed for incandescent lamps. To allow an easy retrofit to an LED light source, without requiring large changes to existing intersection alternating current power distribution and logic circuits, signal assemblies incorporate a power supply to drive LEDs at a lower, controlled, direct current power level. In the past, this has resulted in an LED traffic signal assembly with a separate power supply built on a Printed Circuit Board (PCB) and a separate LED matrix PCB connected via wiring between the two PCB""s as well as spliced into the original incandescent power wiring. Integration of LEDs onto a single PCB including the power supply results in a smaller PCB with corresponding manufacturing and cost of materials benefits.
Cost of materials and assembly time contribute to total cost and therefore to commercial success. Previous LED traffic signals used a large number of total components, each individual component adding material cost, assembly cost and introducing a potential quality control, moisture, and/or vibration failure opportunity.
Traffic signals are susceptible to xe2x80x9csun phantomxe2x80x9d phenomena. When a light source, for example the sun, shines upon the face of a traffic signal, a bright spot, or worse, internal reflection from within the signal, may make it appear to a viewer that the signal is energized when, in fact, it is not, leading to an increased chance for accidents.
Previous incandescent signals have attempted to prevent the xe2x80x9csun phantomxe2x80x9d phenomena by using a visor, internal or external baffles and/or a flat outer face angled towards the ground. Visors and external baffles limit the viewing angle of the signal. Internal baffles add cost to the signal by introducing an element that has no other purpose. Flat outer faces are not allowed, according to some traffic signal specifications which require a spherical front element.
Previous LED signal lamps are especially susceptible to xe2x80x9csun phantomxe2x80x9d phenomena because the rear surface of each LED is highly reflective. Previous LED signal designs located the LEDs on or close to the outer surface where the rear surface of each LED could easily be reached by stray light, creating an increased opportunity for xe2x80x9csun phantomxe2x80x9d reflections. Previous LED signals that use a secondary optical element between the LEDs and the outer cover also suffer from sun phantom effect as the stray light reflects back, generally along the center axis, rather than towards the ground, off of the optical element.
LED signals have an extremely long service life that has increased with each new generation of LEDs. Incandescent lamps, while having a much shorter service life, have relatively constant light output until a total failure occurs, i.e. burnout of the light filament. LED signals, over an extended period, have gradually diminishing light output. Further, LED light output is negatively affected by temperature. In extreme climate or during unnaturally warm periods LED light output diminishes during the day and then returns to a normal level during cooler periods at night.
Because of the difficulty, time and expense of accurately determining when an LED signal has permanently dropped below the acceptable light output limit, it is customary for consumers to automatically replace LED signals upon expiration of the warranty (for example, five years). This may result in years of useful service life being unnecessarily wasted, reducing the cost effectiveness of using LED signals.
U.S. patent application Ser. No. 09/543,240, now abandoned incorporated herein by reference, discloses monitoring circuits for an LED signal that shut off the signal if the power supply or LED arrays change their voltage and/or current characteristics. Unfortunately, LED light output may degrade without a change in the LED signal""s voltage or current characteristics.
Therefore, the present invention has the following objectives:
1. An LED signal which minimizes the problem of xe2x80x9csun phantomxe2x80x9d erroneous signal aspects.
2. An LED signal which presents a uniform brightness display aspect equal to or better than a common incandescent lamp traffic signal.
3. An LED signal that has materials and manufacturing assembly cost advantages.
4. An LED signal comprised of a single printed circuit board carrying both the LEDs and the power supply components.
5. An LED signal retro-fitable into existing incandescent traffic signals, without requiring removal of the existing reflector assembly.
6. An LED signal capable of easy upgrade to higher output LEDs without requiring recalculation of the optical elements.
7. An LED signal with a display aspect unaffected by changes in individual LED light output.
8. An LED signal usable in multiple configurations, each specific to a given application, with a minimum of unique components being required.
9. A cost efficient apparatus for capturing and utilizing horizontal light emissions from common LEDs.
10. A cost efficient apparatus for capturing and utilizing horizontal light emissions from common LEDs, useable with a variable number of LEDs.
11. A cost efficient apparatus for creating a controlled light emission pattern with minimal optical materials cost.
12. A cost effective and automatic means for detecting when an LED signal""s light output has fallen below an acceptable level.
Further objects will be realized by one skilled in the art, through review of the following description and appended claims.
The above objects and other advantages are achieved with the present invention. Placement of the LEDs, to create an overlapping light emission pattern at an increased distance from a Multiple Collimating Zone Element (MCZE) creates a uniform display aspect for the signal, without individual points of light. The increased distance also allows placement of power supply components and circuitry on a single PCB with the LEDs, spaced so as to prevent interference with the LED light.
A light pipe collector (LPC) for LED signals captures and redirects normally unutilized horizontally emitted LED light. Designed for total internal reflection, the LPC redirects horizontally emitted light without the use of mirrored surfaces or reflective coatings. A single LPC may be snap fit to the PCB over a group of several LEDs. As the light output of LEDs increases with each new generation the same LPC may be used with fewer LEDs without requiring redesign and/or remanufacture.
When designed with a side to side dimension in close tolerance with the external dimension of the LEDs, the LPC also assists in properly orienting LEDs that may be misaligned due to imperfections in the LEDs"" housings or poor assembly. The LPC may be designed to provide an optical solution that eliminates or minimizes the need for additional optics in the LED signal. Formed into directional arrows or letters the LPC creates an LED signal with a minimal number of LEDs without requiring other optics. The LPC creates an LED signal with materials, manufacturing and operating cost efficiencies previously unavailable.
The xe2x80x9csun phantomxe2x80x9d phenomena is prevented by a large radius spherical outer distribution cover, angled to reflect stray light away from the viewer, towards the ground. A complex inner surface on the distribution cover creates a shaped light distribution, focused upon the viewer, while at the same time further directing stray light reflections, again, towards the ground.
A light sensor, mounted within an LED signal housing senses the LED light output level. When the light level falls below a preset level, a short circuit is created that breaks a fusible link on the input power line. The broken fusible link disables the LED signal thereby alerting users that replacement is necessary.
Materials, assembly and installation cost efficiencies are also realized by a novel snap together housing design which adds to an overall reduction in total number of components. The signal fits into existing standard incandescent traffic signals upon removal only of the incandescent bulb and original outer lens. Electrical connection is made by merely screwing a socket mating connector into the existing incandescent socket.